You're describing a property of their compilers as evidence that one is a superset and one is not. That TS transforms your code doesn't make it not a superset. It would not be a superset if it didn't support some JS feature. If it didn't support extends or something.
I'm not saying Typescript isn't a superset of JavaScript, I'm saying Flow is more direct superset in a way that's important. I'm more trying to clarify people who are saying Flow and TS are both supersets and therefore directly comparable. Specifically, the root poster's contention was that they're both supersets and therefore both "not JS".
